1. Field of the Invention
The present invention relates to a gas concentration sensor for measuring the concentration of combustible gas, such as vaporized fuel, contained in, for example, intake air to be supplied into an intake pipe of, for example, an internal combustion engine.
2. Description of the Related Art
Conventionally, a fuel supply system for supply of fuel from a fuel tank to an engine includes a first supply system which functions in the following manner. Fuel is pumped from the fuel tank by means of a fuel pump and is then sent to an injector through a fuel pipe.
The fuel supply system further includes a second supply system which functions in the following manner. Vaporized fuel generated within the fuel tank is temporarily adsorbed by a canister. Accumulated vaporized fuel is purged from the canister and is sent as purge gas to an intake pipe.
Accordingly, in addition to fuel injected from the injector, vaporized fuel, such as purge gas, is burned within a cylinder of the engine (hereinafter vaporized fuel is referred to merely as purge gas).
When, as a result of supply to the engine of purge gas in addition to injected fuel, an air-fuel ratio deviates from a theoretical value, the purification capability of a catalyst with respect to CO, HC, and NOx lowers considerably. As a result, CO, HC, and NOx contents of exhaust gas increase.
Accordingly, in order to use purge gas as a portion of main fuel for combustion, for example, at engine start-up, particularly when the catalyst is inactive, optimum control of purge gas supply through highly accurate measurement of purge gas concentration is very important.
A sensor for measuring purge gas concentration may utilize, for example, ultrasonic waves (ultrasonic sensor). Such an ultrasonic sensor has been developed, but a satisfactory ultrasonic sensor has not yet been developed.
Such an ultrasonic sensor detects concentration of purge gas on the basis of propagation time of a transmitted ultrasonic wave. However, such an ultrasonic sensor has the following problem. When a material constituting an ultrasonic element of the ultrasonic sensor deteriorates over time, the characteristics of the ultrasonic element change, so that the propagation time of an ultrasonic wave to be measured changes. Therefore, it has been difficult to accurately measure concentration of purge gas.
Accordingly, there has been a problem that concentration of purge gas is very difficult to control accurately on the basis of results of measurement regarding the concentration of purge gas.
The present invention has been achieved to solve the above problems, and an object of the invention is to provide a gas concentration sensor capable of accurately measuring the concentration of a specific gas, such as purge gas.
To achieve the above object, the invention of claim 1 provides a gas concentration sensor which detects gas concentration of a specific gas in a gas under measurement. The gas concentration sensor comprises an ultrasonic element for transmitting an ultrasonic wave to a reflection surface, which is a predetermined distance away from the ultrasonic element, via a space into which the gas under measurement is supplied. The ultrasonic element is caused to transmit an ultrasonic wave toward the reflection surface. The ultrasonic wave reflects off the reflection surface to thereby become a first reflection wave, the ultrasonic element reflects the first reflection wave so that the first reflection wave is again reflected off the reflection surface. Concentration of the specific gas is detected on the basis of a propagation time of a reflection wave other than the first reflection wave.
During measurement of concentration of a specific gas by use of a so-called ultrasonic sensor, an ultrasonic wave is transmitted into a gas under measurement, a time period until receipt of the transmitted ultrasonic wave or a reflection wave from a reflection surface is measured; and the concentration of the specific gas is obtained on the basis of the time period (propagation time).
This technique utilizes a characteristic of ultrasonic waves that, as shown in FIG. 4, their propagation time changes depending on the concentration of a specific gas contained in a gas under measurement.
However, in the case where characteristics of a molded material of the ultrasonic element have changed due to time-course deterioration, as shown in FIG. 8, the propagation time T1xe2x80x2 of the first reflection wave becomes greater than the propagation time T1 of the first reflection wave as measured in a new sensor. If measurement of the concentration of a specific gas is based on the propagation time T1 of the first reflection wave as measured in the new sensor, gas concentration cannot be determined accurately. By contrast, a reflection wave other than the first reflection wave (for example, a second reflection wave) is merely reflected off the surface of the ultrasonic element and is not affected by the internal structure of the ultrasonic element. As shown in FIG. 8, even when the sensor is deteriorated, the propagation time T2, T2xe2x80x2 of, for example, the second reflection wave exhibits less variation and is less susceptible to deterioration of the sensor than is the propagation time T1 of the first reflection wave.
Therefore, in the present invention, the concentration of a specific gas is determined on the basis of the propagation time of the second or later reflection wave, which is less susceptible to deterioration of the sensor, instead of the propagation time of the first reflection wave, which is more susceptible to deterioration of the sensor.
Thus, gas concentration can be measured accurately. On the basis of accurately measured gas concentration, gas concentration can be adjusted accurately, whereby, for example, control of air-fuel ratio can be performed favorably.
Preferably the gas concentration sensor is further characterized in that the first reflection wave reflected off the reflection surface and the second reflection wave-which results from reflection of the first reflection wave off the reflection surface-are detected; and a time period between the arrival of the first reflection wave and the arrival of the second reflection wave is measured in order to detect the gas concentration of the specific gas.
Accordingly, in order to measure the concentration of the specific gas in a gas under measurement, the ultrasonic element is caused to transmit an ultrasonic wave toward the reflection surface, which is a predetermined distance away from the ultrasonic element, via a space into which the gas under measurement is supplied. The first reflection wavexe2x80x94which results from reflection of the transmitted reflection wave on the reflection surfacexe2x80x94is first detected. Then, the second reflection wavexe2x80x94which results from reflection of the first reflection wave off the ultrasonic element and then off the reflection surfacexe2x80x94is detected. Subsequently, a time period between the arrival of the first reflection wave and the arrival of the second reflection wave (i.e., the propagation time of the second reflection wave) is measured in order to detect the gas concentration of the specific gas.
As described above, even when the sensor deteriorates, the propagation time of the second reflection wave hardly changes. Therefore, the propagation time of the second reflection wave is less susceptible to deterioration of the sensor than is the propagation time of the first reflection wave. Further, since the degree of attenuation of the second reflection wave is smaller than that of later or higher-order reflection waves, the received waveform of the second reflection wave can be detected clearly.
Therefore, in this preferred aspect of the invention, the concentration of the specific gas is determined on the basis of the propagation time of the second reflection wave, which is less susceptible to deterioration of the sensor, instead of the propagation time of the first reflection wave, which is more susceptible to deterioration of the sensor. That is, the concentration of the specific gas is determined through measurement of a time period between the arrival of the first reflection wave and the arrival of the second reflection wave.
Thus, gas concentration can be measured more accurately. On the basis of accurately measured gas concentration, gas concentration can be adjusted accurately, whereby, for example, control of air-fuel ratio can be performed favorably.
Preferably, the gas concentration sensor is further characterized in that a first arrival time between a time of transmission of the ultrasonic wave and the arrival of the first reflection wave and a second arrival time between the time of transmission of the ultrasonic wave and the arrival of the second reflection wave are measured; and the gas concentration is detected on the basis of the difference between the first and second arrival times.
According to this preferred aspect, a first arrival time between a time of transmission of the ultrasonic wave and the arrival of the first reflection wave and a second arrival time between the time of transmission of the ultrasonic wave and the arrival of the second reflection wave are measured. For example, as shown in FIG. 7, a second arrival time (T3) and a first arrival time (T1) is measured. Subsequently, the difference (T2) between the two time periods is calculated. Since the difference represents the propagation time of the second reflection wave, the gas concentration is detected on the basis of the difference.
Preferably, the gas concentration sensor is further characterized in that the measurement of each time period is performed when the level of a received signal of the ultrasonic wave exceeds a predetermined threshold.
The invention according to this preferred aspect exemplifies the timing for measuring a propagation time or arrival time of an ultrasonic wave. Accordingly, a point in time when the level of a received signal of the ultrasonic wave exceeds a predetermined threshold is used as a timing for measuring a propagation time or arrival time of an ultrasonic wave. Thus, it becomes possible to measure a propagation time or arrival time of an ultrasonic wave through use of simple means.
The term xe2x80x9cpropagation timexe2x80x9d refers to a time period required for propagation of an ultrasonic wave over a certain distance. The term xe2x80x9carrival timexe2x80x9d refers to a time period between a measurement start timing serving as a reference and a timing at which a certain reflection wave arrives at the sensor. For the first reflection wave, the propagation time and the arrival time become the same.
Preferably, the gas concentration sensor is further characterized in that an ultrasonic wave involving at least two frequency components is transmitted; and a propagation time required for propagation of the ultrasonic wave over a predetermined distance is measured through utilization of the modulation point present in the ultrasonic wave in order to detect the concentration of the specific gas.
This preferred aspect of the invention exemplifies the timing for measuring a propagation time of an ultrasonic wave. As shown in FIG. 7, an ultrasonic wave having at least two frequency components is transmitted. Subsequently, a time period (T1) between the modulation point in the transmitted wave and the modulation point in the first reflection wave, and a time period (T3) between the modulation point in the transmitted wave and the modulation point in the second reflection wave are measured. This enables precise measurement of the propagation time of the ultrasonic wave without regard for the degree of intensity of a signal.
Preferably, the gas concentration sensor is further characterized in that an ultrasonic wave involving at least one antiphase component is transmitted; and a propagation time required for propagation of the ultrasonic wave over a predetermined distance is measured through utilization of the antiphase point present in the ultrasonic wave in order to detect the concentration of the specific gas.
This preferred aspect of the invention exemplifies the timing for measuring a propagation time of an ultrasonic wave. As shown in FIG. 10, an ultrasonic wave having at least one antiphase point is transmitted. Subsequently, a propagation time which the ultrasonic wave requires to propagate over a predetermined distance is measured through utilization of the antiphase point present in the ultrasonic wave. Thus, without performance of frequency modulation, precise measurement of the propagation time of the ultrasonic wave can be performed without regard for the degree of intensity of a signal.
Preferably, the gas concentration sensor is further characterized in that the specific gas is vaporized fuel for use with an internal combustion engine.
This preferred aspect of the invention specifies a specific gas to be measured by the gas concentration sensor. The gas concentration sensor is intended to measure the concentration of vaporized fuel, such as purge gas. Since fuel gas concentration can be measured accurately, an air-fuel ratio, etc. can be controlled favorably.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.